Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure of the element to light followed by development by uniformly heating the element. These elements also include thermographic elements in which an image is formed by imagewise heating the element. By far the most common imaging compositions included in thermally processable elements are those that contain an organic silver salt to provide a source of silver ion relied upon for image formation and a thermally activated reducing agent for the silver ion. The silver ion can form a metallic silver image when reduced (as is usually practiced in forming black-and-white images) and/or the thermally activated reducing agent when oxidized in the oxidation-reduction reaction can form a dye image (as is usually practiced in forming color images). A summary of these common types of photothermographic element constructions is provided by Research Disclosure, Vol. 170, June 1978, Item No. 17029. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. Many of these photothermographic element constructions are also useful as thermographic elements. Most photothermographic elements that rely on photosensitive silver halide to catalyze an image-forming oxidation-reduction reaction can be used as thermographic elements. When use exclusively as a thermographic element is contemplated, a common modification is to omit the photosensitive silver halide and to rely on the imagewise application of heat to drive the image-forming oxidation-reduction reaction, as illustrated by Grant U.S. Pat. No. 3,080,254.
It is common practice to include a protective coating overlying the imaging layer(s) in the construction of a thermally processable element. In addition to the variety of functions that protective coatings are recognized to perform in imaging elements generally, such as adhesion to the underlying portion (i.e., layer or support) of the element, optical transparency as required (including resistance to fingerprints and scratches), low transport friction, low self-adhesion (necessary for use of spool wound or stacked sheet elements), and sensitometric compatibility with the imaging layers, the protective coatings of thermally processed elements are also commonly relied upon to resist deformation during thermal processing and to reduce or prevent loss of volatile components during thermal processing. Also, unlike imaging elements that rely on penetration by aqueous processing solutions, the protective coatings of thermally processable elements need not be water permeable and often serve their purpose better when relatively impermeable. In thermally processable elements imaging layer protective overcoats are often referred to as barrier layers. As a result of differing functional requirements, most conventional selections of protective coatings for thermally processable elements have taken specialized forms unsuited for imaging elements generally.
Research Disclosure, Item No. 17029, cited above, XI. Overcoat Layers, catalogues known overcoat components of photothermographic elements. Surfactants are specifically addressed in section X. Coating Aids. The term "surfactant", a condensation of the phrase "surface active agent", designates a compound that contains a hydrophilic moiety and a hydrophobic moiety. Surfactant 10G.TM., available from Olin Co., a non-ionic surfactant, is specifically identified in Item 17029. Przezdziecki U.S. Pat. No. 4,741,992, assigned to Eastman Kodak Co., discloses the use of Surfactant 10G.TM. in the overcoat of a thermally processable element.
Surfactants containing a fluorocarbon hydrophobic moiety have been used in the overcoat of thermally processable elements. For example, Mack et al U.S. Pat. No. 5,198,406, assigned at issue to Polaroid Corp., discloses the overcoat incorporation of Zonyl FSN.TM., available from DuPont.
Hirai et al U.S. Pat. No. 5,736,293, assigned at issue to Fuji Photo Film Co., discloses photothermographic elements having a protective coat containing the following surfactants: ##STR1## This surfactant combination appears in numerous protective coat formulations disclosed in patents assigned at issue to Fuji Photo Film Co.
Yonkoski et al U.S. Pat. No. 5,380,644, Simpson et al U.S. Pat. No. 5,545,507, and Murray (et al) U.S. Pat. Nos. 5,635,339, 5,654,130 and 5,686,228, assigned at issue to Minnesota Mining and Manufacturing Co. (a.k.a. 3M) or Imation Corp. (a company formed by 3M to continue this line of business), disclose varied protective coats for thermally processable elements. Only Yonkoski et al identifies any protective coat component, a linear polymer with a pendant fluorocarbon moiety, as being a surfactant. Yonkoski et al includes the surfactant to control surface anomalies--specifically mottle. The term "mottle", not defined by Yonkoski et al, is generally employed in the imaging arts to designate image noise significantly in excess of that attributable to granularity. Simpson et al and Murray (et al) demonstrate that surfactants are not essential surface coating components and are, in fact, often omitted entirely where other components permit.
An extensive listing of surfactants for use in photographic elements is contained in Research Disclosure, Vol. 389, September 1996, Item 38957, IX. Coating physical property modifying addenda, A. Coating aids.